The present invention relates to a linear actuator for a machine, particularly a bending press.
More precisely, the invention relates to a linear actuator according to the pre-characterising portion of claim 1.
From U.S. Pat. No. 4,693,131 a device is known for effecting a linear advance, the device having a fixed base on which a table is mounted for sliding. A male screw is carried for free rotation by the base and is rotatable by a first motor. The male screw engages a female screw which is carried for free rotation by the movable table and is rotatable by a second motor. The two motors can be rotated in the same sense at slightly different speeds to achieve a slow approach stroke or in opposite senses to achieve a fast stroke.
Other linear actuators with male-and-female screw mechanisms in which the male and female screws are associated with respective motors are described in U.S. Pat. Nos. 4,614,128 and 4,703,666.
In many machines, there is a need to cause a tool (for example, the die of a bending press) to perform a linear movement constituted by two clearly distinct stages: an approach stroke, during which the tool must be moved at a high speed of translation with a low thrust (sufficient to overcome the friction, the inertia and possibly the weights of the movable bodies), and a working stroke with exactly the opposite characteristics, that is, a high thrust (in bending presses this may even be of the order of hundreds of tonnes) and a low speed of translation.
The known actuators mentioned above have been found unsuitable for use in a machine of this type. In fact, if the motor for effecting the approach stroke were dimensioned to supply only the low thrust necessary during this stroke, its torque would be insufficient to counteract the high reaction torque acting on the male-and-female screw mechanism during the operation of the motor which operates the working stroke. If the known actuators mentioned above were used in applications for which markedly different thrusts are required, the motor effecting the approach stroke would have to be oversized.
From GB-A-714,003 a metal-cutting machine is known having a fixed structure carrying a movable part. A low-speed motor is fast to the movable part and a high-speed motor is fast to the fixed structure. A female screw is rotatably mounted in the movable part and a male screw--which is in mesh with the female screw--is rotatably mounted in the fixed structure and is driven from the high-speed motor. The female screw is driven from the low-speed motor. An electromagnetic brake has a rotor which is fast to the male screw and has the purpose of holding the male screw against rotation during a low-speed working stroke of the movable part. The device shown in GB-A-714,003 could not be applied to a bending press because of its bulky structure.